Patent Document 1 below shows a document stacker with an observation window for visibly confirming a kind or denomination of the stacked document in a safe and easy way. This document stacker is advantageous because it allows a person to visually confirm a kind of an uppermost document stacked in a stacker through a transparent member secured to a part of a frame in the stacker from outside of the frame without unlocking a lock device provided. A pushing plate for urging documents into a compartment in the stacker has its length and width both shorter than those of the document to show a periphery of the top document exposed out of the circumference of the pushing plate in contact with the top bill within the compartment. The document stacker also comprises a rod inserted from outside through a hole formed in the frame or a handle protruded from the pushing plate or a backing plate outside the frame through the hole to manually move the uppermost document to the position for visually confirming the document through the transparent member.